1. Field of the Invention
The present invention relates to a capacitive ultrasonic probe device to be inserted into a body cavity and used for ultrasonic diagnosis.
2. Description of the Related Art
In recent years, ultrasonic probe devices have been widely employed for acoustic diagnosis. Also, these ultrasonic probe devices are usually employed with piezoelectric elements taking advantage of a piezoelectric phenomenon, but nowadays, capacitive ultrasonic probe devices employing a capacitive micromachined ultrasonic transducer (c-MUT) have been proposed.
On the other hand, in recent years, harmonic imaging diagnosis employing high-frequency signals is becoming standard diagnostic modality since a distinct diagnostic image can be obtained, which cannot be obtained with the conventional B mode diagnosis.
The harmonic imaging diagnostic method can be classified into (1) a tissue harmonic imaging method wherein a harmonic wave superimposed upon the fundamental ultrasonic wave in response to influence of nonlinearity of organism tissue at the time of an ultrasonic wave propagating within a body is separated using various types of methods, and an image is formed by employing this signal, and (2) a contrast harmonic imaging method wherein a contrast medium bubble is injected into a body, a harmonic wave which occurs at the time of the bubble being ruptured by irradiation of a transmitting ultrasonic wave is received, the harmonic wave superimposed upon the fundamental ultrasonic wave is separated by various types of methods, and an image is formed by employing this signal.
It has been understood that both of those excel in S/N, which cannot be obtained by conventional B mode tomography, and can obtain diagnostic images with suitable resolution. Both of those have contributed in improvement in medical diagnostic precision.
As an example of the ultrasonic transducers employed for conventional external harmonic imaging diagnostic devices, two-way ultrasonic transducers which can be used for fundamental wave transmission and harmonic wave reception have been employed. Note that an arrangement may be made wherein the echo of an ultrasonic pulse reflected from organism tissue is received at an ultrasonic transducer provided separately from an ultrasonic transducer for transmission.
It is necessary to effectively remove fundamental wave components relating to deterioration of harmonic images since the signal level of a harmonic wave signal is extremely small as compared to the fundamental wave. Therefore, a known harmonic wave component extraction technique (particularly, a second harmonic wave component extraction technique) has been employed.
As described above, as for an ultrasonic transducer, a capacitive ultrasonic transducer obtained by processing a silicon semiconductor substrate using silicon micromachine technology has been proposed, in addition to conventional piezoelectric type ultrasonic transducers.
Capacitive ultrasonic transducers have been disclosed in PCT Japanese Translation Patent Publication No. 2004-503312, and PCT Japanese Translation Patent Publication No. 2004-503313 as conventional examples. These Publications have disclosed capacitive ultrasonic probe devices which aim at external use.
Note that there is a capacitive micromachined ultrasonic transducer configured by layering ultrasonic transducers (c-MUT), such as disclosed in U.S. Pat. No. 6,558,330.
In order to employ the harmonic imaging technology, it is necessary to obtain an ultrasonic transducer including broadband properties, and the capacitive ultrasonic transducers are appropriate for harmonic imaging diagnosis due to the broadband properties thereof.